1. Field of the Invention
The present invention relates generally to a brake actuating apparatus for a vehicle, and more particularly to an actuating apparatus of a type in which a hydraulic pressure applied to a brake is controlled by a motor.
2. Related Art
Conventionally, a vehicle has employed an auxiliary brake actuating system together with a brake pedal depressing force for achieving so called an anti-lock and traction controls. Various types of a brake actuating apparatus acting as the auxiliary brake actuating system have conventionally been known as disclosed, for example, in Unexamined Japanese Patent Application (OPI) No. Hei. 1-266050.
FIG. 1 is a schematic view showing the fundamental structure of the conventional brake actuating apparatus taught in the OPI No. Hei. 1-266050. The conventional brake actuating apparatus shown in FIG. 1 is provided with a power cylinder 54 which is connected through solenoid plungers 52, 53 to an accumulator 50 into which brake pressure is accumulated by an electric pump 51. The power cylinder 54 supplies hydraulic pressure to a brake caliper 55 of each of brake cylinders of the vehicle wheels through a brake pipe 56. The solenoid plungers 52, 53 are independently controlled by a signal from a central control unit (not shown) to thereby control the pressure to the brake caliper 55.
By the way, it is more preferable to independently control each of the vehicle wheels. In the conventional brake actuating apparatus described above, however, since a common hydraulic pressure is applied to all the four wheels, it is difficult to appropriately control the brake pressure. Further, the brake pipes connecting the accumulator to each of the brake cylinders and also the solenoid plungers for the brake control are required, which causes the apparatus to be bulky and intricate. Further, since the conventional brake actuator of this type is spaced apart from the brake caliper, the reduction in its size is so limited.
On the other hand, another conventional art U.S. Pat. No. 4,809,824 teaches to eliminate the drawbacks accompanying the afore-described conventional apparatus, as shown in FIG. 2. FIG. 2 is a sectional view showing a brake actuating apparatus disclosed in the U.S. Pat. No. 4,809,824. In this apparatus, a driving force of an electric motor 40 is transmitted to a ball screw 44 through a speed reducer 43 and a solenoid clutch 41 so that a slider 45 which engages with the ball screw 44 urges against a friction pad 46.
The latter conventional apparatus, however, still requires a bulky motor 40 and speed reducer 43 for obtaining a sufficient brake force, which cause the apparatus to be large in size. Further, the apparatus is provided with the solenoid clutch 41 for compensating the deterioration of the responsibility due to the bulk in components and intricacy in structure, which causes the structure more bulky and intricate and would increase load to a spring.
Furthermore, in the conventional apparatus shogun in FIG. 2, a depressing force applied to or stroke of the brake pedal is detected by a sensor (not shown), and an electric control device outputs electric current corresponding to an output signal of the sensor to the motor 40 to drive the brake actuating apparatus. This type of brake actuating apparatus is so called as a by-wire type. The by-wire type of the brake actuating apparatus is in fact advantageous with respect in that it is easy to be applied to an anti-skid brake system and/or traction control system since the electric signal is applicable to various kinds of control. Further, since the control structure of the brake force is relatively simple compared to the conventional apparatus as described above the utility demand of this type of brake actuating apparatus may more increase in future. However, this type of brake actuating apparatus suffers from a problem that the actuating apparatus would not be controlled when occurring an unexpected electrical defect due to a fault of the sensor or a breakdown of electric wires, since the by-wire type brake actuating apparatus electrically controls the whole the operational force for the brake apparatus in accordance with the electric signal supplied from the sensor detecting the depressing force applied to or stroke of the brake pedal, as different from the former conventional mechanical brake operating structure employing the hydraulic pressure or mechanical link system. Accordingly, the conventional brake actuating apparatus would raise a serious problem in safety.